Geotechnical News • June 2015
35
GEOSYNTHETICS
design, the assessment of a candidate
geotextile for a proposed filtration
application must address a series of
material properties. A value for each
material property is typically obtained
from laboratory index tests performed
in accordance with the appropriate
national or international standard test
method. When specifying a geotextile
for a filter application, our primary
interest lies in determining an index
value for the pore size opening, the
permittivity and the strength of the
fabric (see Fig. 1). Consider now the
measurement and reporting of each of
these index properties.
Pore size opening of the
geotextile
The manufacturing process exerts
a significant influence on the pore
structure of a geotextile. A woven
geotextile exhibits a relatively uniform
pore structure that is characterized
by openings of nearly constant size
and little spatial variability across the
surface of the roll. The weave pattern
may differ between products, as may
the size of the polymer strands used
to provide the materials of the warp
and the weft, both of which govern the
size of the resulting openings between
the strands. In contrast, a nonwoven
geotextile exhibits a relatively wide
range of opening sizes which vary spa-
tially across the surface of the roll. As
a result, the pore-size distribution is
strongly influenced by factors includ-
ing the type and density of strands
and, in the case of a needle-punched
product, the needle-shape, punch
density and direction or, in the case of
a heat-bonded product, the nature of
the contact surface and line-speed of
the heated rollers (Bhatia and Smith,
1996). Accordingly, the two main
types of geotextile, namely woven and
nonwoven, exhibit a significantly dif-
ferent pore structure.
Index test methods used to determine
the pore size opening of a geotex-
tile involve (i) variations on reverse
sieving of grains through the pores of
the fabric (by means of a dry siev-
ing, a wet sieving or a hydrodynamic
sieving method, as illustrated in Fig.
2), (ii) the injection or removal of a
fluid from the pores of the fabric (by
mercury intrusion porosimetry, or a
bubble point method, respectively)
and (iii) direct image analysis of the
pore space. Current design practice is
based on methods of reverse-sieving,
for which manufacturers report values
in their technical literature.
Reverse-sieving methods are suitable
for determining the largest pore size
openings, which are believed to exert
the greatest influence on base soil
retention in filtration applications. In
dry sieving (see for example, ASTM
D4751), different size fractions of
glass beads (from small to large, in
ascending sequence of size fraction)
are sieved through the geotextile
by means of a shaking action. The
surface of the fabric is pre-treated
with an anti-static spray to minimize
the influence of attraction resulting
from static electricity. In wet siev-
ing (see for example, ISO 12956),
a graded mixture of glass beads is
sieved by shaking in combination with
a continuous spray of water, following
pre-treatment of the fabric by a wet-
ting agent. In hydrodynamic sieving
(see for example, CAN CGSB 148.1
No.10), a graded mixture of glass
beads is sieved by repeated immersion
of the geotextile in water and hence
alternating flow conditions, without
any shaking action.
A common aspect of all three stan-
dard test methods (see Fig. 2), is that
a gradation analysis of beads passing
through the fabric during a standard-
ized duration of shaking or number
of immersion cycles is used to infer
the size of the largest pore openings
in the geotextile. The three methods
yield similar but not identical values
of opening size (see Table 1). Gener-
ally, the dry sieving method yields
a relatively larger value of pore size
than that obtained from either wet
sieving or hydrodynamic sieving (see
for example, Faure et al., 1986; Van
der Sluys and Dierickx, 1990; Bhatia
et al., 1996). Accordingly, when using
design criteria for soil retention that
relate a characteristic opening size of
the geotextile to a characteristic grain
size of the base soil, it is important to
recognize not only the empirical origin
of the design criterion but also the cor-
relation to a particular laboratory test
method to determine the opening size
of the geotextile.
Tabel 1. Variation of pore size opening (μm) with sieving technique
(extracted from Van der Sluys and Dierickx, 1990)
Geotextile
O
n
(μm)
Dry
sieving
Wet
sieving
Hydrodynamic
sieving
W3
O
90
O
98
278
348
301
387
282
374
W4
O
90
O
98
354
416
307
358
303
360
W6
O
90
O
98
294
339
259
295
225
289
W7
O
90
O
98
253
260
172
210
194
224
NW1
O
90
O
98
179
202
143
195
133
181
NW3
O
90
O
98
204
236
145
191
150
202
NW4
O
90
210
189
150
Note: Woven (W) geotextile: Nonwoven (NW) geotextile
